1. Field of the Invention
The present invention relates to measurement of arterial blood flow in the limb of a living being, and particularly to an improved apparatus and method for measuring total net blood flow and determining an accurate zero flow level adapted for use with a non-invasive type of flowmeter responsive only to pulsatile flow.
The normal flow of blood through the arteries of a living being can be divided into variable and steady components. The variable component usually is called pulsatile flow, modulated flow, or AC flow (by analogy to electric current). The steady component is called non-modulated, residual, or DC flow. The pulsatile flow component can be further subdivided into forward flow (outward from the heart to the extremities) which occurs during the systolic period of the heart cycle, and reverse (retrograde) flow, which may occur during the diastolic period.
Due to the elasticity of the blood vessels, there is typically a small, fairly steady residual forward flow during the post-diastolic period. This constant, or non-modulated, residual flow can comprise a significant percentage of the total net forward flow during each heart cycle. The total net forward flow is the algebraic sum of the pulsatile systolic and steady residual forward flows and the pulsatile diastolic reverse flow.
Researchers and physicians investigating the condition of patients suffering from heart or circulatory impairment are usually interested in both the quantitative total net forward blood flow and in the blood flow waveform, plotted as a function of time. The total net forward flow is a measure of the blood perfusing the limb during one complete heart cycle. The blood flow waveform, on the other hand, is useful for analyzing various dynamic characteristics of the circulatory system. For greatest usefulness, it is important that the waveform provide quantitative information, including an accurate zero-flow level.
2. Description of the Prior Art
Non-invasive blood flowmeters of the type described in U.S. Pat. No. 3,659,591; No. 3,759,247; and No. 3,809,070 (assigned to the assignee of this invention) have many advantages stemming from the use of sensing electrodes placed on the skin instead of subcutaneously by surgical implantation. However, offset voltages are developed between the electrode surfaces and the skin. These offset voltages tend to drift with time and mask the signal contribution of steady flow components. Consequently, such instruments are not responsive to steady flow, only to variations in flow rate, which is the pulsatile part of the flow. In fact, the non-invasive, constant-magnetic-field type of blood flowmeter perceives the residual flow amplitude during the post-diastolic period as a "pulsatile zero" reference. The indicated total net flow per full cycle, therefore, may be less than the actual systolic forward portion of the pulsatile flow component. In other words, the previously mentioned drift problems prevent determination of the true zero flow level on the blood flow waveform produced by such a flowmeter.